The need sometimes arises to synchronize or encode data in a variable rate stream. Specifically, for example, the need arises in magnetic recording apparatus to synchronize or encode digital data for single-track, self-clocking formats.
The widespread use of magnetic recording techniques for digital data involves several different single-track format schemes as explained to some extent in U.S. Pat. No. 3,962,726. One popular format is referred to as F/2F recording in a class sometimes referred to as the Manchester codes. Other formats in which both binary data and timing information are carried by a recorded single signal are sometimes collectively referred to as non-F/2F codes.
The system of the present invention is applicable for both F/2F codes and non-F/2F codes of recording. Accordingly, the system of the present invention is applicable in a variety of systems for generating or recovering a variable-rate data stream, as in conjunction with magnetic recording apparatus.
Dynamic magnetic recording and reproducing techniques generally involve time-space relationships. That is, magnetic discontinuities are recorded along a length of magnetic medium. In sensing the medium, representative signal fluctuations are displaced in time in a direct relationship to the spacing of the magnetic discontinuities. However, assuming a direct time-space relationship presumes a constant relative speed between the magnetic medium and the sensing apparatus, e.g. the magnetic transducer head. Thus, variations in the speed of the relative motion between a length of magnetic tape and a transducer head will vary the timing relationship between signal fluctuations. In precision equipment, the critical relative speed may be quite consistent; however, in other apparatus the relative speed may vary widely producing a variable rate data stream. For example, apparatus is in widespread use for sensing a recording from a magnetic stripe on a card as it is moved manually. Such equipment requires a system for synchronizing a variable rate data stream and specifically for sensing the stream to distinguish between the signal fluctuations representative of data and those fluctuations representing synchronizing signals. Specifically, the need is for a system to sample a variable rate data stream during periods when data is manifest.
Recapitulating to some extent, magnetic stripe cards are frequently used manually in association with a transducer apparatus as for sensing or recording. Such systems must have considerable tolerance for speed variations as well as record deterioration to provide reliable intelligence. The present invention affords a reliable system for synchronizing such a data stream.
In general, synchronizing systems for variable data streams have used the time interval between the most recent synchronizing pulses as a basis for predicting the time interval between current synchronizing pulses. Although a variety of such techniques have been developed and are in widespread use, it is apparent that a radical change in the period between sync signals may cause such systems to fall out of synchronization. Such radical changes are not unusual in manual-drive equipment or where a portion of the recorded data is obliterated.
In general, the system of the present invention anticipates data signals on the basis of several prior synchronizing intervals so that the recent history of operation (rather than the past cycle) provides the basis for current observations. An estimate of the current interval is registered. If the current interval is found to coincide with the estimate (to a reasonable tolerance) the estimate is not changed. If the estimate is found to be too large or too small, it is altered by a fixed percentage of its value, e.g. three percent.
As a specific example, in the F/2F format, a timer whose time is equal to the estimated interval (bit time) is started by each synchronizing transition received. The bit is tested at a time value of 3/4 estimated to confirm or deny the presence of the conditional transition, as per well known technique.
For non-F/2F format, a timer whose time is equal to the estimated interval time is started by each received transition. If no transition is received, the timer repeats itself continuously, until a transition is received to re-synchronize. The bit is tested at a time value of 1/2 of estimated to confirm the level as high or low, as per well known technique.
In the disclosed embodiment of the present invention, the data synchronizer receives a variable rate stream to provide both data and synchronizing signals. The synchronizing signal defines variable intervals which are necessary to interpret the data. A timing means defines short sub-intervals of time which are of variable duration. A counter is associated with the timing apparatus for counting a predetermined number of the sub-intervals to define a variable sync interval. Apparatus is incorporated for varying the periods of the sub-intervals under control of the historical rate of the stream from the input source.